For a long time I thought these were seed pods of some kind. But since they were on this Black Oak tree, which reproduces with acorns, it didn’t make sense. I now find that they are products of the Oak Apple Gall Wasp (Amphibolips confluenta). The life cycle of this insect is very interesting, although scientists are still learning about it. Adult wasps hatch from galls in June and July. Males and females mate and then drop to the ground. Female wasps burrow into the soil at the base of the tree and inject eggs into the roots. Wasp larvae hatch and munch on the roots for over a year before becoming pupae (resting stage). Only wingless female wasps hatch from the pupae underground. These females crawl out of the soil and up the tree trunk in early Spring. They find a newly-growing leaf and inject an egg into the mid-rib (center vein). The larvae that hatch inside the leaf are small and round. As they grow, they cause a chemical reaction inside the leaf that forms a gall around the larvae. The gall itself is actually a mutated leaf. Each larva continues eating and growing, and the apple gall grows with it (these were about 2 inches in diameter). Apple galls get their name because large galls look a little bit like apples.